364 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 364–365† Kinetics and Mechanism of Chromium(III)-catalysed Oxidation of Formic Acid by Cerium(IV) in Aqueous Sulfuric Acid Media† Premendra Nath Saha, Sudhin K. Mondal, Dalia Kar, Mahua Das, Asim K. Das* and Rajani K. Mohanty* Department of Chemistry, Visva-Bharati, Santiniketan - 731 235, West Bengal, India In the CrIII-catalysed oxidation of formic acid by CeIV in aqueous sulfuric acid media, an intermediate involving the oxidant, substrate and catalyst is formed and a CrIII/CrIV catalytic cycle operates.The kinetics and mechanism of RuIII- and IrIII-catalysed oxidation of formic acid in aqueous sulfuric acid media have been reported1,2 by us recently. CrIII is known3 to catalyse the title reaction and this prompted us to carry out the present investigation in detail in continuation of our studies1,2,4 on metal-ion catalysis in CeIV oxidation. In fact, there are only a very few cases where the kinetics and mechanistic aspects of CrIII catalysis in CeIV oxidation have been reported5 so far.Here we report the kinetics and mechanism of the title reaction in 1.0 mol dmµ3 H2SO4 under the conditions [CeIV]T=(2.5–4.5)Å10µ3 mol dmµ3, [Cr]T=(0.25– 7.0)Å10µ2 mol dmµ3 and [HCO2H]T=(0.25–3.0) mol dmµ3 in the temperature range 40–60 °C. Experimental Materials and Reagents.·Standard stock solutions of CeIV and HCO2H were prepared as reported1 earlier. The CrIII catalyst was in the form of chromium(III) potassium sulfate (BDH, AR), Cr2(SO4)3.K2SO4.24H2O, and the stock solution in aqueous sulfuric acid media was standardised as usual.All other reagents were of reagent grade. Procedure and Kinetic Measurements.·The method employed for following the progress of the reaction has been discussed earlier.1 The reactions were followed up to 80–85% completion and the pseudo-first-order rate constants (kobs) were computed from the linear plot (ra0.99) of log [CeIV] vs.time. The rate constants (kobs) were reproducible within �3–5%. Stoichiometry and Product Analysis.·Stoichiometry determination of the title reaction under the experimental conditions was carried out as reported earlier.1 The observation conforms to: CrIII 2CeIV+HCO2Hh2CeIII+CO2+2H+ (1) The concentration of CrIII remains unchanged after the reaction. Results and Discussion Dependence on [CeIV].·The rate of disappearance of CeIV shows a first-order dependence on [CeIV] up to 80-85%, then the plot (ln [CeIV] vs.t) slightly deviates. The pseudo-firstorder rate constant (kobs) is independent of the initial concentration of CeIV in the range (2–6)Å10µ3 mol dmµ3. µ d[CeIV] dt =k[CeIV] or µ d ln [CeIV] dt =kobs (2) Dependence on [CrIII].·The pseudo-first-order rate constant (kobs) at fixed [HCO2H] (=1.0 mol dmµ3) increases sharply with increasing [CrIII] but levels off at higher values of [CrIII]. It can be represented as: kobs= A[Cr]T B+C[Cr]T (3) or 1 kobs = B A[Cr]T + C A (4) At fixed [HCO2H] in 1.0 mol dmµ3 H2SO4, A, B and C are constants and expressed in terms of different rate constants as discussed later on.[Cr]T gives the total concentration of chromium added as catalyst. The constants are estimated from linear plots (ra0.98) of 1/kobs vs. 1/[Cr]T. Dependence on [HCO2H].·At fixed [Cr]T, kobs shows a first-order dependence on [HCO2H] in 1.0 mol dmµ3 H2SO4. kobs=ks[HCO2H]T (5) The values of ks are: 104 ks/dm3 molµ1 sµ1=2.2�0.1 (45 °C), = 3.5�0.1 (50 °C), =7.0�0.1 (60 °C) at [Cr]T=[K+] =0.01 mol dmµ3, [CeIV]T=4Å10µ3 mol dmµ3, [H2SO4] =1.0 mol dmµ3, [HCO2H]T=(0.25–3.0) mol dmµ3.Dependence on [HSO4 µ].·For variations in [HSO4 µ] over the range (0.3–1.75) mol dmµ3 at a fixed [H+], the composition of the mixture [H2SO4]+[HClO4]3[H+] =1.75 mol dmµ3 was varied.4 This leads to [HSO4 µ]3[H2SO4], ignoring the dissociation of HSO4 µ. [HSO4 µ] shows a rate-retarding effect on kobs (see Table 1).The plot of 1/kobs vs. [HSO4 µ] is linear (r=0.99) with positive intercept and slope. Dependence on [H+].·For variations in [H+] over the range (0.3–1.75) mol dmµ3 at a fixed [HSO4 µ], the composition of the mixture, [H2SO4]+[NaHSO4]3[HSO4 µ] =1.75 mol dmµ3, was varied6 assuming [H+]3[H2SO4]. The dependence on [H+] is expressed from an experimental fit as: kobs=k0+kHp[H+]+kHP[H+]2 (6) Because of the existence of so many proton-dependent equilibria4a among the reactants, the said approximation can be called into question.4a In fact, because of this complexity4a in the present reaction media, no attempt was made to explain the observed [H+] dependence from the proposed reaction scheme.Acrylonitrile Polymerisation Test.·When acrylonitrile was added to the reaction mixture under a nitrogen atmosphere, on prolonged standing polymerisation starts very slowly. However, in the absence of the title catalyst, addition of acrylonitrile under identical conditions leads very rapidly to the reaction mixture becoming viscous.Effect of MnSO4, Products and Other Factors.·MnSO4 (ca. 0.01 mol dmµ3) can itself catalyse the reaction. CrIII (ca. 0.01 *To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Table 1 Dependence of kobs on [HSO4 µ] for CrIII-catalysed oxidation of formic acid by CeIV.[HCO2H]T=1.0 mol dmµ3, [CeIV]T=4Å10µ3 mol dmµ3, [H+] =1.75 mol dmµ3, [Cr]T=[K+] =0.01 mol dmµ3, 50 °C [HSO4 µ]/mol dmµ3 10µ2 kobs µ1/s 0.3 6.6 0.5 8.4 0.75 10.8 1.0 13.2 1.5 18.6 1.75 22.2J. CHEM. RESEARCH (S), 1997 365 mol dmµ3) which is a catalyst in the title investigation has been found to antagonise the catalytic activity of MnSO4. No discernible effects of [Na+] and/or [K+] (up to 0.07 mol dmµ3) or of the product, [CeIII] (up to 4Å10µ3 dmµ3), were observed.Ambient light and aerial oxygen had no effect on kobs. Mechanism of the Reaction.·The observations in 1.0 mol dmµ3 H2SO4 can be mathematically represented as: kobs= a[Cr]T[HCO2H]T b+c[Cr]T (7) The following reaction scheme (S=HCO2H) can explain the experimental findings: k1 CeIV+SMCeIV(S) (complex C1) (8) kµ1 k2 C1+CrIIIMCeIV(S)µCrIII (outer-sphere complex, C2) (9) kµ2 k3 C2hCeIII(S) CrIV (inner-sphere complex, C3) (10) fast C3hCeIII(S.)CrIII+H+ (11) fast CeIII(S.)CrIII+CeIVh2CeIII+CO2+H++CrIII (12) Under the steady-state conditions of the species C1 and C2, the above scheme under the reasonable approximation [Cr]T2[CrIII] leads to the rate equation: kobs=µ d ln [CeIV] dt = 2k1k2k3 f[HCO2H]T[Cr]T {kµ1kp+k2k3[Cr]T} (13) where f gives the fraction of the total cerium(IV), [CeIV]T, which is kinetically active, and kp=kµ2+k3.Combining eqns. (3)–(7) and (13) gives A=2k1k2k3 f[HCO2H]T. B=b=kµ1kp, C=c=k2k3, a=2k1k2k3 f and k5= 2k1k2k3 f[Cr]T {kµ1kp+k2k3[Cr]T} (14) In the given scheme, C2 is an outer-sphere complex due to the inherent inertness7 of CrIII, but C3 is an inner-sphere complex where the CrIV centre generated is a labile7 one.In C3, electron transfer occurs rapidly to produce an intermediate complex in which a formate radical (S.), being tightly bound to CrIII, is not available in the bulk sufficiently to initiate polymerisation. This can explain qualitatively the sluggish rate of polymerisation in the presence of CrIII. From the plot of 1/kobs vs. 1/[Cr]T at fixed [HCO2H]T the composite constants, kn (=2k1 f ) and Km=k2k3/kµ1kp were evaluated. The values are: 104 kn/sµ1=2.4�0.2 (40 °C), 3.4�0.2 (45 °C), =4.4�0.1 (50 °C), with activation parameters DH‡=144 kJ molµ1 and DS‡=1µ174 J Kµ1 molµ1; and 10µ2 Km/dm3 molµ1=0.95�0.15�0.1 (45 °C), =3.6�0.1 (50 °C), at [HCO2H]T=[H2SO4] =1.0 mol dmµ3. The antagonistic activity of the mixed catalyst system, MnII- +CrIII, indirectly supports the involvement of CrIV in CrIII catalysis.It is known8 that MnSO4 rapidly removes CrIV thus hindering the catalytic activity of CrIII. In fact, participation of the catalytic cycle CrIII/CrIV in CeIV oxidation in aqueous sulfuric acid media has also been reported5 previously. CrIII is an inert centre7 while CeIV is a relatively more labile one.7 Consequently, the equilibria leading to different sulfato species of CeIV are only important in the present kinetics to explain the [HSO4 µ] dependence.Under the experimental conditions of aqueous sulfuric acid media, the important CeIV species are9 Ce(SO4)2+, Ce(SO4)2 and HCe(SO4)3 µ. By considering the relative values9 of Q1, Q2 and Q3 which are the successive formation equilibrium constants for the species Ce(SO4)2+, Ce(SO4)2 and HCe(SO4)3 µ, respectively, [Ce(SO4)2] can be reasonably given4b by eqn. (15). [Ce(SO4)2]2 [CeIV]T 1+Q3[HSO4 µ] =f[CeIV]T (15) Use of eqn. (15) in eqn.(13) affords eqn. (16) after rearrangement. 1 kobs = 1 p + Q3[HSO4 µ] p (16) where p= 2k1k2k3[HCO2H]T[Cr]T kµ1kp+k2k3[Cr]T Eqn. (16) explains the hydrogensulfate dependence. From the plot of 1/kobs vs. [HSO4 µ] (r=0.99), where [HSO4 µ] =(0.3–1.75) mol dmµ3 at fixed [HCO2H]T=1.0 mol dmµ3, [Cr]T=[K+] =0.01 mol dmµ3, the estimated Q3=3.1 at 50 °C conforms well to the reported values.4b,9b Previously, in many cases, Ce(SO4)2 has been identified4b,9b as the kinetically active CeIV species.Thanks are due to CSIR, New Delhi, for financial assistance. Received, 20th January 1997; Accepted, 9th June 1997 Paper E/7/00449D References 1 A. K. Das and M. Das, J. Chem. Soc., Dalton Trans., 1994, 589. 2 A. K. Das and M. Das, Indian J. Chem., 1995, 34A, 866. 3 (a) N. N. Sharma and R. C. Mehrotra, Anal. Chim. Acta, 1954, 11, 417; 1955, 13, 419; (b) N. N. Sharma and R. C. Mehrotra, Z. Anal. Chem., 1960, 173, 395. 4 (a) A. K. Das, J. Chem. Res., 1996, (S) 185; (M) 1023; (b) A. K. Das and M. Das, Int. J. Chem. Kinet., 1995, 27, 7. 5 (a) A. Chimatadar, S. T. Nandibewoor, M. I. Sambrani and J. R. Raju, J. Chem. Soc., Dalton Trans., 1987, 573; (b) S. R. Kampli, S. T. Nandibewoor and J. R. Raju, Indian J. Chem., 1990, 29A, 908. 6 G. Arcoleo, G. Calvaruso, F. P. Cavasino and C. Sbriziolo, Inorg. Chim. Acta, 1977, 23, 227. 7 cf. R. G. Wilkins, in The Study of Kinetics and Mechanism of Reactions of Transition Metal Complexes, Allyn & Bacon, Boston, 1974. 8 (a) W. Watanabe and F. H. Westheimer, J. Chem. Phys., 1949, 61, 17; (b) L. Kaplan, J. Am. Chem. Soc., 1955, 77, 5469. 9 (a) L. T. Bugaenko and H. Kuan-Lin, Russ. J. Inorg. Chem., 1963, 8, 1299 and references cited therein; (b) S. K. Misra and Y. K. Gupta, J. Chem. Soc. A, 1970, 2